linux-kernel - Re: [PATCH next] i2c: Fix end of loop test in i2c_atr_find_mapping_by

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Message-ID: <a22d74b9-06b1-4a4b-9c06-4b0ff7f9b6c2@stanley.mountain>
Date: Wed, 23 Apr 2025 20:29:51 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Romain Gantois <romain.gantois@...tlin.com>
Cc: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
	Luca Ceresoli <luca.ceresoli@...tlin.com>,
	Wolfram Sang <wsa+renesas@...g-engineering.com>,
	Andi Shyti <andi.shyti@...nel.org>, linux-i2c@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH next] i2c: Fix end of loop test in
 i2c_atr_find_mapping_by_addr()

On Wed, Apr 23, 2025 at 05:25:44PM +0200, Romain Gantois wrote:
> Hello Dan,
> 
> On Wednesday, 23 April 2025 10:21:18 CEST Dan Carpenter wrote:
> > When the list_for_each_entry_reverse() exits without hitting a break
> > then the list cursor points to invalid memory.  So this check for
> > if (c2a->fixed) is checking bogus memory.  Fix it by using a "found"
> > variable to track if we found what we were looking for or not.
> 
> IIUC the for loop ending condition in list_for_each_entry_reverse() is
> "!list_entry_is_head(pos, head, member);", so even if the loop runs to 
> completion, the pointer should still be valid right?
> 

head is &chan->alias_pairs.  pos is an offset off the head.  In this
case, the offset is zero.  So it's &chan->alias_pairs minus zero.

So we exit the list with c2a = (void *)&chan->alias_pairs.

If you look how struct i2c_atr_chan is declareted the next struct member
after alias_pairs is:

	struct i2c_atr_alias_pool *alias_pool;

So if (c2a->fixed) is poking around in the alias_pool pointer.  It's not
out of bounds but it's not valid either.

regards,
dan carpenter